A prior art immersed membrane water filtration system is shown in FIG. 1. An open tank (a) holds a membrane module (b) immersed in tank water (c). Feed water to be filtered flows into the tank, typically continuously. Suction on an inner surface of the membranes in the membrane module (b) draws filtered permeate through the membrane wall. Solids are rejected by the membranes and accumulate in the tank water (c). Solids rich retentate is continuously or periodically drained from the tank.
The membrane module (b) is cleaned in part by backwashing and aeration. In backwashing, a backwashing liquid (typically permeate or permeate with a chemical additive) is pumped into the inner spaces of the membranes and flows into the tank water (c). In aeration, air bubbles are created at an aerator (d) mounted below the membrane module (c). The air bubbles agitate and scour the membranes and create an air lift effect. The air lift effect moves tank water (c) in a recirculation pattern (e) upwards through the membrane module (b) and in a downcomer (f) through spaces between the perimeter of the module (b) and the sides of the tank (a). The tank water (c) flowing in the recirculation pattern (e) further physically cleans the membranes and disperses solids rich water from near the membrane module (b).